1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor laser capable of operating at a high output power, which is preferably used in the fields of communication, laser medical treatment, laser beam machining, laser printers and the like.
2. Description of the Related Art
FIG. 6 is a view showing the configuration of an example of a self-aligned structure semiconductor laser with a separate confinement heterostructure (hereinafter, such a laser is referred to as an SCH-SAS LD). The laser is reported in IEEE Journal Quantum. Electronics., Vol. 29, No. 6, (1993) p1889-1993.
Referring to FIG. 6, a cladding layer 2 of n-AlGaAs, a quantum well active layer 5 of GaAs/AlGaAs, a cladding layer 9 of p-AlGaAs, and a contact layer 10 of p-GaAs are sequentially formed on an n-GaAs substrate 1. A current blocking layer 7 of n-AlGaAs is embedded in the cladding layer 9.
In the self-aligned structure semiconductor laser shown in FIG. 6, the current blocking layer 7 having a stripe-like window and a bandgap wider than that of the cladding layer 9, i. e., a refractive index lower than that of the cladding layer is embedded. Therefore, a refractive index difference is formed also in a direction (lateral direction) parallel to the quantum well active layer 5, so that laser light can be confined also in the lateral direction of the stripe. As a result, two dimensional real index structure is realized.
Japanese Unexamined Patent Publication JP-A 62-73687(1987) discloses a self-aligned structure semiconductor laser in which upper and lower cladding layers are respectively formed on both faces of an active layer, a current blocking layer is formed on the upper cladding layer, a center portion of the current blocking layer is then removed away to form a stripe-like groove, and a third cladding layer is embededly grown.
Japanese Unexamined Patent Publication JP-A 4-370993(1992) discloses a self-aligned structure semiconductor laser in which a refractive index difference is formed also in the lateral direction of a stripe by a current blocking layer having a refractive index lower than that of a cladding layer, and, in order to facilitate the regrowth of a stripe-like window of the current blocking layer, an optical guide layer is disposed between an active layer and the current blocking layer.
A thesis (Applied Physics Letters., Vol. 37, No. 3, (1980), p262-263) reports a self-aligned structure semiconductor laser in which a current blocking layer is made of a material having a bandgap narrower than that of an active layer, and laser light is laterally confined by optical absorption of the current blocking layer.
FIG. 7 is a view showing the configuration of an example of a self-aligned structure semiconductor laser with a perfect SCH (hereinafter, such a laser is referred to as a PSCH-SAS LD). This laser is disclosed in International Patent Publication WO96/12328 in the name of the assignee of the present application.
Referring to FIG. 7, a cladding layer 2 of n-AlGaAs, an optical guide layer 3 of n-AlGaAs, a carrier blocking layer 4 of n-AlGaAs, a quantum well active layer 5 of GaAs/AlGaAs, a carrier blocking layer 6 of p-AlGaAs, an optical guide layer 8 of p-AlGaAs, a cladding layer 9 of p-AlGaAs, and a contact layer 10 of p-GaAs are sequentially formed on an n-GaAs substrate 1. A current blocking layer 7 of n-AlGaAs is embedded in the optical guide layer 8.
In such a PSCH-SAS LD also, a refractive index difference is formed also in the lateral direction by the existence of the current blocking layer 7, and hence two dimensional real index structure is realized.
FIGS. 8A, 8B and 8C are views illustrating an example of a conventional method of fabricating the SCH-SAS LD. First, as shown in FIG. 8A, the cladding layer 2 of n-AlGaAs, the quantum well active layer 5 of GaAs/AlGaAs, and a part of the cladding layer 9 of p-AlGaAs are sequentially crystal-grown on the n-GaAs substrate 1. An n-AlGaAs layer 7a to be the current blocking layer 7 is then uniformly crystal-grown.
Next, as shown in FIG. 8B, a mask is formed in the lateral sides of a region where the center stripe-like window is to be formed, and the stripe-like window is opened in the n-AlGaAs layer 7a by wet etching by which crystals are not damaged, thereby forming the current blocking layer 7. Thereafter, the mask is removed away.
Next, as shown in FIG. 8C, the remaining part of the cladding layer 9 is crystal-grown, and the contact layer 10 of p-GaAs is then crystal-grown.
FIGS. 9A, 9B and 9C are views illustrating an example of a conventional method of fabricating the PSCH-SAS LD. First, as shown in FIG. 9A, the cladding layer 2 of n-AlGaAs, the optical guide layer 3 of n-AlGaAs, the carrier blocking layer 4 of n-AlGaAs, the quantum well active layer 5 of GaAs/AlGaAs, the carrier blocking layer 6 of p-AlGaAs, and a part of the optical guide layer 8 of p-AlGaAs are sequentially crystal-grown on the n-GaAs substrate 1. An n-AlGaAs layer 7a to be the current blocking layer 7 is then uniformly crystal-grown.
Next, as shown in FIG. 9B, a mask is formed in the lateral sides of a region where the center stripe-like window is to be formed, and the stripe-like window is opened in the n-AlGaAs layer 7a by wet etching by which crystals are not damaged, thereby forming the current blocking layer 7. Thereafter, the mask is removed away.
Next, as shown in FIG. 9C, the remaining part of the optical guide layer 8 is crystal-grown, and the cladding layer 9 of p-AlGaAs and the contact layer 10 of p-GaAs are then sequentially crystal-grown.
In such a SCH-SAS LD and a PSCH-SAS LD, in order to realize lateral light confinement and suppression of current spread so as to attain excellent single lateral mode oscillation, the current blocking layer must be located at a position close to the active layer as much as possible, and the width of the window through which a current passes must be formed so as to accurately coincide with the designed value.
In the conventional fabrication method, in the etching step of forming the stripe-like window in the current blocking layer, overetching in which even the active layer is etched away frequently occurs, thereby fabricating a problem in that a high yield cannot be attained.
As a technique that etching is controlled to a desired depth while preventing such overetching from occurring, known is a method in which an etching stop layer for automatically chemically stopping etching is formed below the current blocking layer. In the method, however, only the etching controllability in the depth direction is improved and the controllability in the lateral direction, i.e., the controllability of the window width of the current blocking layer is not improved. Since the window width of the current blocking layer affects the oscillation threshold and the stability of the lateral mode, the method using an etching stop layer is not sufficient for solving the problem.